Phosphating process



United States Patent 3,144,360 PHOSPHATING PROCESS Bert E. Palm, Cleveland, Ohio, assignor to The Lubrizol Corporation, Wicltlilfe, Ohio, a corporation of Ohio No Drawing. Filed Feb. 19, 1962, Ser. No. 174,291 11 Claims. (ill. 148-615) The present invention relates, as indicated, to a phosphating process. In a more particular sense, it relates to a process for rapidly phosphating a ferrous metal article by means of an aqueous phosphating solution characterized principally by its high total acidity.

It is known in the metal finishing art to provide metal surfaces, especially ferrous surfaces, with an inorganic phosphate coating by contacting them with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves as an excellent base for the later application of organic coatings such as paint, lacquer, varnish, primers, synthetic resins, enamels, and the like.

Such inorganic phosphate coatings are generally formed on a metal surface by means of aqueous solutions which contain small amounts of the phosphate ion and, option ally, certain auxiliary ions, including metallic ions such as sodium, manganese, zinc, cadmium, iron, copper, lead, nickel, cobalt, and antimony ions, and non-metallic ions such as ammonium, chloride, bromide, nitrate, and chlorate ions. These auxiliary ions modify the character of the phosphate coating and adapt it for a wide variety of applications.

The preparation and use of aqueous phosphating solutions is well-known in the metal finishing art as shown by US. Patents 1,206,075; 1,247,668; 1,305,331; 1,485,025; 1,610,362; 1,980,518; 2,001,754; and 2,875,111.

Aqueous phosphating solutions are generally prepared by dissolving in water small amounts of phosphoric acid and, optionally, at least one metal salt such as a nitrate, phosphate, nitrite, sulfate, chloride, or bromide of sodium, manganese, zinc, cadmium, iron, nickel, copper, lead, or antimony. Ordinarily an oxidizing agent such as sodium chlorate, potassium perborate, sodium nitrate, ammonium nitrate, sodium chlorite, potassium perchlorate, or hydrogen peroxide is included in the phosphating solution to depolarize the metal surface being treated and thereby increase the rate at which the phosphate coating is formed on the metal surface. Other auxiliary agents such as anti-sludging agents, coloring agents, and metal cleansing agents may also be incorporated in the phosphating solu tion. One common type of commercial phosphating bath which contains zinc ion, phosphate ion, and a depolarizer is made by dissolving small amounts of zinc dihydrogenphosphate, sodium nitrate, and phosphoric acid in water.

In all such known phosphating solutions, the total acidity is found to vary broadly from about 5 to a maximum of about 80 points, with most solutions having a total acidity between 20 and 60 points. The term points total acidity as employed in the phosphating art represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a milliliter sample of a phosphating solution in the presence of phenolphthalein as an indicator. As the total acidity of a phosphating solution rises, an undesirable pickling or corrosive attack of the solution on the metal being treated becomes more and more pronounced. Thus, workers in the metal finishing art have avoided using phosphating solutions having a total acidity greater than about 80 points because of the strong pickling properties and poor coating characteristics believed to be inherent in all such solutions.

In accordance with the present invention it has been discovered, quite unexpectedly, that aqueous phosphating solutions having very high total acidity, ranging from about 90 to about 850 points, and containing critical amounts of certain characterizing metallic and non-metallic ions are ideally suited for the rapid phosphating of ferrous metal articles. The use of such solutions under certain process conditions to be explained in detail hereinafter makes it possible to form a satisfactory phosphate coating on a ferrous article in as short a time as one second. The commercial applications of such a rapid phosphating process are manifold. For example, it renders economically feasible the continuous phosphating of cold-rolled strip steel at speeds consonant with those employed in modern, high-production rolling mills. It is also well adapted for the continuous phosphating of hotor cold-rolled plate steel stock such as that used in the manufacture of automobile bumpers. It is also well adapted for the phosphating of galvanized ferrous articles and it is intended that the term ferrous metal article as used herein be inclusive of a galvanized ferrous metal article.

It is, therefore, an object of the present invention to provide a process for the rapid phosphating of a ferrous metal article.

Another object is to provide a phosphated ferrous metal article, the phosphate coating of which is integrally bonded to the metal substrate and which serves as an excellent base for the application of an organic coating composition.

These and other objects will become apparent as the description of the invention proceeds.

In its broadest aspect, then, the present invention relates to a process for rapidly phosphating a ferrous metal article which comprises introducing said article for about 1 to about 20 seconds into a hot aqueous phosphating solution having a total acidity within the range from about to about 850 points and containing as essential ingredients from about 1.5 to about 8 percent of zinc ion, from about 3.5 to about 20 percent of phosphate ion, from about 5 to about 26 percent of nitrate ion, and from about 1 to about 4.5 percent of calcium ion; all percentages being by weight.

In a more particular sense, the present invention concerns a process for rapidly phosphating a ferrous metal article to form thereon an integral phosphate coating varying from about 25 to about 1000 milligrams per square foot of surface area which comprises introducing said article for about 1 to about 12 seconds into an aqueous phosphating solution maintained at 190240 F., having a total acidity within the range from about to about 300 points, and containing as essential ingredients from about 2 to about 4 percent of zinc ion, from about 5 to about 10 percent of phosphate ion, from about 8 to about 16 percent of nitrate ion, and from about 1.4 to about 3 percent of calcium ion.

Best results from the standpoint of economy and excellence of the phosphate coating are realized by means of a process for rapidly phosphating a ferrous metal article to form thereon an integral phosphate coating varying from about 50 to about 5-00 milligrams per square foot of surface area which comprises introducing said article for about 5 to about 10 seconds into an aqueous phosphating solution maintained at 200-2'12 F., having a total acidity within the range from about to about 225 points, and containing as essential ingredients about 2.7- 3.3 percent of zinc ion, about 7-8 percent of phosphate ion, about 8-12 percent of nitrate ion, about 1.52 percent of calcium ion, and about 0.6-1.0 percent of ammonium 1on.

In addition to the characterizing zinc, phosphate, ni-- to about 1 percent. Similarly, the chloride ion will be used in an amount varying from about 0.5 to about 3.5 percent, preferably from about 1 to about 1.5 percent.

To exhibit the rapid phosphating action required for the purposes of this invention, the aqueous phosphating solution must be hot, e.g., at least about 150 F. The temperature of the solution preferably should be at least about 190 F. and is most desirably within the range l90240 F. Since the hereindescribed phosphating solutions are highly concentrated, temperatures as high as 230 F. can be realized at atmospheric pressure. By the use of superatmospheric pressures, temperatures as high a 250 F., 300 F., or more can be obtained, if desired. Generally, however, it is preferred to use the phosphating solution under atmospheric pressure in the interest of convenience and economy.

It is likewise important that the ferrous article be immersed in the phosphating solution. Spray-phosphating techniques, with their attendant high heat loss, are not well adapted for the purposes of the present invention. The manner of immersing the ferrous article is not critical; that is, it can be the relatively quiescent kind of immersion exemplified by dipping techniques or the continuously moving kind of immersion exemplified by the 'unmersion of moving steel strip stock in the phosphating solution by means of submerged rollers or other devices which serve the same purpose. Similarly, plates of heavy gauge steel may be conveyed continuously through the phosphating solution.

In the accepted practice of phosphating a metal article, the surface thereof is usually cleaned by physical and/ or chemical means such as immersion in or spraying with an aqueous alkali-base cleanser, mechanical abrading or polishing, vapor degreasing, etc. The cleansed article is then ordinarily rinsed with water before immersion in the phosphating solution so as to preclude contamination of the solution. Upon completion of the phosphating operation, the phosphated article is rinsed, optionally, with water and/or a hot dilute aqueous solution of chromic acid containing from about 0.01 to about 0.2 percent of CrO The chromic acid rinse appears to seal the phosphate coating and improve its utility as a base for the application of paint, lacquer, varnish, and the like. In lieu of the dilute aqueous chromic acid, dilute aqueous solutions of metal chromates, metal dichromates, chromic acid-phosphoric acid mixtures, and chromic acid-metal dichromate mixtures may be used. Any or all of such known pro-treatments and post-treatments can be employed, if desired, in the practice of the present invention.

As stated hereinbefore, the phosphating process of this invention forms a phosphate coating varying in weight from about to about 1000 milligrams, generally from about to about 500 milligrams, per square foot of surface area in about 1 to about 20 seconds. In order to minimize pickling of the metal surface and assure an ad- TABLE I.-PIIOSPHATING SOLUTION The phosphating solutions required for the purposes of this invention can be made conveniently by first dissolving zinc nitrate, calcium nitrate, and, optionally, ammonium dihydrogen phosphate and zinc chloride, in sufi'icient water to yield the required weight percentages of the several ions and then adjusting the acidity of the solution (i.e., the points total acid) by the addition of phosphoric acid and/ or nitric acid. In lieu of the salts specified, one may use the corresponding bases and mineral acids such as zinc oxide, calcium oxide, calcium hydroxide, ammonium hydroxide, phosphoric acid, nitric acid, and hydrochloric acid. Alternately, the solutions can be made by dissolving zinc dihydrogenphosphate, calcium nitrate, and, optionally, ammonium chloride in water and then adjusting the acidity of the solutions by the addition of phosphoric acid and/ or nitric acid.

Thus, it is apparent that the ions of the phosphating solutions used in the practice of this invention may be derived from a variety of compounds and it appears to be of little consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts selected to provide the required ions, the resulting solution is effective to serve the purposes of this invention. It is necessary only that these salts or acids be used in amounts to provide the necessary concentration of the required characterizing ions. In addition to the characterizing ions present in the phosphating solution, certain supplementary ions such as bromide, chlorate, perchlorate, nitrite, or perborate ions may also be present to increase the rust inhibiting qualities of the coating, reduce sludging, etc.

The presence of the calcium ion serves to suppress the formation of massive, hydrated crystalline coatings and yield instead a highly desirable micro-crystalline or amorphous coating. The nitrate ion serves as an oxidizing agent to depolarize the metal surface and increase the coating speed of the phosphating solution. Its presence is likewise essential in the phosphating solutions employed for the purpose of the present invention.

The preparation of certain of the phosphating solutions shown in Table I is carried out as follows.

SOLUTION A 18.4 grams of zinc oxide, 60.6 grams of 100 percent phosphoric acid, and grams of water are thoroughly mixed. To this mixture there is added 76 grams of calcium nitrate trihydrate, 24.8 grams of zinc nitrate, and 859 grams of water.

SOLUTION B This solution is prepared by dissolving 45.2 grams of zinc nitrate, 17.0 grams of percent phosphoric acid, 26.3 grams of ammonium dihydrogenphosphate, and 41 grams of anhydrous calcium nitrate in 871 grams of water.

SOLUTION C 65.5 grams of zinc nitrate, 24.5 grams of 100 percent phosphoric acid, 38.1 grams of ammonium dihydrogenphosphate, and 59.3 grams of anhydrous calcium nitrate are dissolved in 812 grams of water.

SOLUTION D 79.6 grams of zinc nitrate, 29.9 grams of 100 percent phosphoric acid, 46.3 grams of ammonium dihydrogenphosphate, and 72.2 grams of anhydrous calcium nitrate are dissolved in 771 grams of Water.

D E H 01 Points total acid.

I K L MI N l SOLUTION E 101 grams of zinc nitrate, 38 grams of 100 percent phosphoric acid, 59 grams of ammonium dihydrogenphosphate, and 92 grams of anhydrous calcium nitrate are dissolved in 710 grams of water.

SOLUTION F 120 grams of zinc nitrate, 45.1 grams of 100 percent phosphoric acid, 70 grams of ammonium dihydrogenphosphate, and 109 grams of anhydrous calcium nitrate are dissolved in 655 grams of water.

SOLUTION G 146 grams of zinc nitrate, 54.9 grams of 100 percent phosphoric acid, 85.2 grams of ammonium dihydrogenphosphate, and 133 grams of anhydrous calcium nitrate are dissolved in 5 81 grams of water.

SOLUTION H 180 grams of zinc nitrate, 67.4 grams of 100 percent phosphoric acid, 104.5 grams of ammonium dihydrogenphosphate, and 163 grams of anhydrous calcium nitrate are dissolved in 485 grams of Water.

SOLUTION I 196 grams of zinc nitrate, 73.5 grams of 100 percent phosphoric acid, 114 grams of ammonium dihydrogenphosphate, and 177.5 grams of calcium nitrate are dissolved in 439 grams of water.

SOLUTION J 38.8 grams of zinc nitrate, 24.8 grams of 100 percent phosphoric acid, 36.7 grams of ammonium dihydrogenphosphate, 45.3 grams of anhydrous calcium nitrate, and 17.8 grams of Zinc chloride are dissolved in 837 grams of water.

SOLUTION K 53.6 grams of zinc nitrate, 34.1 grams of 100 percent phosphoric acid, 50.6 grams of ammonium dihydrogenphosphate, 62.4 grams of anhydrous calcium nitrate, and 24.6 grams of Zinc chloride are dissolved in 775 grams of water.

SOLUTION L 27.3 grams of zinc nitrate, 17.4 grams of 100 percent phosphoric acid, 25.8 grams of ammonium dihydrogenphosphate, 54.4 grams of anhydrous calcium nitrate, and 12.5 grams of zinc chloride are dissolved in 862.6 grams of water.

SOLUTION M 44.5 grams of zinc nitrate, 16.7 grams of 100 percent phosphoric acid, 25.9 grams of ammonium dihydrogenphosphate and 40.3 grams of anhydrous calcium nitrate, are dissolved in 872.5 grams of water.

SOLUTION N i Example 1 Three 4-inch x 12-inch panels of 20-gauge SAE 1010 cold-rolled steel were cleaned by immersion for minutes at 205 F. in an aqueous cleanser compounded from water plus 8 oz./ gal. of a commercial alkali-base cleanser, water-rinsed for 5 seconds at 175 F., and then immersed for 3, 4, and 5 seconds, respectively, in phosphating Solution A. After having been phosphated, the panels were rinsed with water for 10 seconds at room temperature and then rinsed with a dilute aqueous solution of chromic acid (0.25 gram of CrO /1iter) for 5 seconds at room temperature. An examination of the panels showed the coating weights thereon to be as follows.

Coating weight, Immersion time, seconds: nag/ft.

Example I 2 Two 4-inch x 12-inch panels of ZO-gauge SAE 1010 cold-rolled steel were cleaned and rinsed in the manner set forth in Example 1, immersed for 5 seconds at 213 F. in phosphating Solution B, and then water-rinsed and aqueous chromic acid-rinsed in the manner set forth previously. An examination of the panels showed the coating weights thereon to be 209 and 275 mg./ft. respectively.

Example 3 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 215 F. in phosphating Solution C. The coating weights on the panels were 222 and 22-5 mg./ft. respectively.

Example 4 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 215 F. in phosphating Solution D. The coating weights on the panels were found to be 217 and 225 mg./ft. respectively.

Example 5 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 215 F. in phosphating Solution E. The coating weights on the panels were found to be 243 and 250 mg./ft. respectively.

Example 6 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 216 F. in phosphating Solution F. The coating weights on the panels were found to be 252 and 261 mg./ft. respectively.

Example 7 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 220 F. in phosphating Solution G. The coating weights on the panels were found to be 225 and 246 mg./ft. respectively.

Example 8 An experiment similar to that described in Example 2 was carried out, except that the two steel panels were immersed for 5 seconds at 228 F. in phosphating Solution H. The coating weights on the panels were found to be 290 and 301 mg./ft. respectively.

Example 9 An experiment similar to that described in Example 2 was carried out, except that a single steel panel was immersed for 5 seconds at 200 F. in phosphating Solution I. The coating weight thereon was found to be 85.5 mg./ft.

Example 10 A One 4-inch x 12-inch panel of 26-gauge SAE 1010 cold-rolled steel was cleaned and rinsed in the manner set forth in Example 1 and then immersed for one second at 198 F. in phosphating Solution K. The coating weight on the panel was found to be 83.3 mg./ft.

Example 11 Three 4-inch x 8-inch panels of 26-gauge electrical-grade silicon steel were cleaned and rinsed in the manner set forth in Example 1 and then immersed for 4, 10, and 16 seconds, respectively, in phosphating Solution I at 205- 210 F. Thereafter, the panels were rinsed with water and aqueous chromic acid in the manner described in Example 1. An examination of the panels showed the coating weights thereon to be as follows.

Immersion time, seconds: Coating weight, mg./ft.

Example 12 Three 4-inch X 8-inch panels of 26-gauge electricalgrade silicon steel were cleaned and rinsed in the manner set forth in Example 1 and then immersed for 4, 9, and seconds, respectively, in phosphating Solution K at 205 210 F. Thereafter, the panels were rinsed with water and aqueous chromic acid in the manner described in Example 1. An examination of the panels showed the coating weights thereon to be as follows.

Immersion time, seconds: Coating weight, mg./ft.

1 Pickling of the steel during the 20-second immersion period is believed to be responsible for the lower coating weight than that observed for the 0-second immersion period.

Example 13 Residence Time in the Coating Line Speed, ftJminute Phosphating Weight, Solution, trig/it.

Seconds Example 14 Plates of heavy gauge (0.125 inch thick) low carbon steel measuring 48 inches by 80 inches were mechanically abraded with finely-divided alumina in the presence of an aqueous alkaline cleanser (water plus 8 oz./gal. of a commercial alkali-base cleanser), flushed with water, and then conveyed continuously by means of rollers through a phosphating solution maintained at 205 210 F., having a total acidity of about 200 points, and containing 2.9 percent of zinc ion, 7.4 percent of phosphate ion, 11.4 percent of nitrate ion, 1.9 percent of calcium ion, and 0.8 percent of ammonium ion. The residence time of the plates in the phosphating solution was about 8 seconds. The emerging plates were then rinsed with water at 150-160 F., air-dried, and coated with a rolling mill lubricant. The plates were found to have an adherent,

light-gray phosphate coating weighing 225 mg./ft. 7

What is claimed is:

1. A process for rapidly phosphating a ferrous metal article which comprises introducing said article for about one to about 20 seconds into a hot aqueous phosphating 8 solution having a total acidity within the range from about to about 850 points and containing as essential ingredients from about 1.5 to about 8 percent of zinc ion, from about 3.5 to about 20 percent of phosphate ion, from about 5 to about 26 percent of nitrate ion, and from about 1 to about 4.5 percent of calcium ion.

2. A process in accordance with claim 1 further characterized in that the aqueous phosphating solution is maintained at a temperature of at least about 190 F.

3. A process in accordance with claim 1 further characterized in that the aqueous phosphating solution additionally contains from about 0.4 to about 2 percent of ammonium ion.

4. A process in accordance with claim 3 further characterized in that the aqueous phosphating solution additionally contains from about 0.5 to about 3.5 percent of chloride ion.

5. A process for rapidly phosphating a ferrous metal article to form thereon an integral phosphate coating varying from about 25 to about 1000 milligrams per square foot of surface area which comprises introducing said article for about one to about 12 seconds into an aqueous phosphating solution maintained at 190240 F., having a total acidity within the range from about to about 300 points, and containing as essential ingredients from about 2 to about 4 percent of zinc ion, from about 5 to about 10 percent of phosphate ion, from about 8 to about 16 percent of nitrate ion, and from about 1.4 to about 3 percent of calcium ion.

6. A process in accordance with claim 5 further characterized in that the ferrous metal article is cold-rolled steel strip stock and in that said steel strip stock is continuously moving while immersed in the aqueous phosphating solution.

7. A process in accordance with claim 5 further characterized in that the aqueous phosphating solution additionally contains from about 0.6 to about 1 percent of ammonium ion.

8. A process for rapidly phosphating a ferrous metal article to form thereon an integral phosphate coating varying from about 50 to about 500 milligrams per square foot of surface area which comprises introducing said article for about 5 to about 10 seconds into an aqueous phosphating solution maintained at 200212 F., having a total acidity within the range from about to about 225 points, and containing as essential ingredients about 2.7-3.3 percent of zinc ion, about 7-8 percent of phosphate ion, about 8-12 percent of nitrate ion, about 1.5-2 percent of calcium ion, and about 0.61.0 percent of ammonium ion.

9. A process in accordance with claim 8 further characterized in that the ferrous metal article is heavy gauge plate steel stock and in that said plate steel stock is continuously moving while immersed in the aqueous phosphating solution.

10. A metal article which has been provided with an integral phosphate coating according to the process set forth in claim 1.

11. An aqueous phosphating solution having a total acidity within the range from about 130 to about 300 points and containing as essential ingredients from about 2 to about 4 percent of zinc ion, from about 5 to about 10 percent of phosphate ion, from about 8 to about 16 percent of nitrate ion, and from about 1.4 to about 3 percent of calcium ion.

References Cited in the file of this patent UNITED STATES PATENTS 2,859,145 Sommers et al Nov. 4, 1958 2,884,351 Cavanagh et a1. Apr. 28, 1959 FOREIGN PATENTS 168,640 Austria July 10, 1951 

1. A PROCESS FOR RAPIDLY PHOSPHATING A FERROUS METAL ARTICLE WHICH COMPRISES INTRODUCING SAID ARTICLE FOR ABOUT ONE TO ABOUT 20 SECONDS INTO A HOT AQUEOUS PHOSPHATING SOLUTION HAVING A TOTAL ACIDITY WITHIN THE RANGE FROM ABOUT 90 TO ABOUT 850 POINTS AND CONTAINING AS ESSENTIAL INGREDIENTS FROM ABOUT 1.5 TO ABOUT 8 PERCENT OF ZINC ION, FROM ABOUT 3.5 TO ABOUT 20 PERCENT OF PHOSPHATE ION, FROM ABOUT 5 TO ABOUT 26 PERCENT OF NITRATE ION, AND FROM ABOUT 1 TO ABOUT 4.5 PERCENT OF CALCIUM ION. 